In recent years, endoscopes (objective lenses) capable of performing magnifying observation have been in high demand in the medical field in order to make accurate diagnoses of lesions. Magnifying observation with an endoscope is performed such that some lenses in the objective lens are moved to change the focal distance, thus changing the observation distance. For example, when the endoscope is focused at a distance of about 5 mm for normal observation, the lenses are moved to change the focal distance, thereby making it possible to focus the endoscope at a distance of up to about 2 mm. As a result, by changing the focal distance from 5 mm to 2 mm, an object can be observed in magnified form, thus allowing magnifying observation. This configuration of lenses is generally called a focusing lens, which is different from zoom lenses that are used in cameras.
As such endoscope objective lenses capable of performing magnifying observation, endoscope optical systems that have a configuration formed of three lens groups, namely, positive, negative, and positive lens groups, and that move the second lens group, thereby performing variable magnification and focusing, are disclosed in PTLs 1 and 2.